1. Field of the Invention
The field of the invention is that of unipolar semiconductor lasers, particularly that useful for generating wavelengths in the 4-12 xcexcm mid-infrared range.
2. Discussion of the Background
The operation of this type of laser is illustrated in FIG. 1. Such a unipolar laser is produced from a stack of layers of semiconductor materials, of calibrated thickness so as to produce quantum-well structures having discrete energy levels. This type of laser has already been described in the literature, and especially in the following references: F. Capasso, A. Y. Cho, J. Faist, A. L. Hutchinson, S. Luryi, C. Sirtori and D. L. Sivco xe2x80x9cUnipolar semiconductor laserxe2x80x9d EP 95 302112.8. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson and A. Y. Cho, Quantum cascade laser, Science, vol. 264, p. 553, J. Faist, F. Capasso, C. Sirtori, D. L. Sivco, J. N. Baillarjeon, A. L. Hutchinson, S. N. G. Chu and A. Y. Cho xe2x80x9cHigh power mid-infrared (xcexxcx9c5 xcexcm) quantum cascade laser operating above room temperaturexe2x80x9d, Appl. Phys. Lett., vol. 68, p. 3680. C. Sirtori, J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson and A. Y. Cho, xe2x80x9cQuantum cascade laser with plasmon enhanced waveguide operating at 8-4 xcexcm wavelengthxe2x80x9d, Appl. Phys. Lett., vol. 66, p. 3242. Unipolar lasers, which are generally cascaded lasers, involve transitions between discrete energy levels within the conduction band, that is to say for example between the levels E1 and E2 illustrated in FIG. 1. When a flux of electrons is injected into the structure, during the transition of the electrons from the energy level E2 to the energy level E1 there is an emission of photons hxcexd=E2xe2x88x92E1. The energy levels involved in this type of structure thus generate wavelengths in the mid-infrared, which is difficult to obtain using other more conventional methods.
At the present time, in order to achieve power control, or even amplitude modulation, in such lasers, a purely electrical control method is used which consists in injecting a relatively large flux of electrons into the said laser.
In electrical modulation, it can be difficult to obtain very high modulation rates. The power supplies able to rapidly switch the high currents for supplying these lasers are in fact xe2x80x9ctop-of-the-rangexe2x80x9d power supplies.
This is why the invention proposes an optical method of controlling a unipolar laser. This method uses optical control beams of much shorter wavelength than the wavelength of the unipolar laser and therefore of a frequency that can be modulated very rapidly.
More specifically, the subject of the invention is a method of controlling a unipolar semiconductor laser comprising a stack of semiconductor layers so as to create a quantum-well structure having, in at least one of the semiconductor layers called an active layer, at least a first discrete energy level E1 and a second discrete energy level E2 in the conduction band, so as to create an electrically excited laser emission with a photon energy corresponding to the difference in energy levels between the said first energy level and the said second energy level, characterized in that it comprises the optical pumping of the said active layer or of another layer in the stack by optical means emitting at least one control beam at a wavelength, of photon energy greater than or equal to the band gap of the optically pumped layer.
In general, semiconductor lasers are lasers having a large number of optical modes, as shown schematically in FIG. 2. One way of making such lasers monomode consists in creating, within the active layer, a diffraction grating whose period fixes the emission wavelength and therefore the mode. At the present time, the diffraction grating may be obtained by a grating etched into the structure as the article by J. Faist et. al. in Proceedings of the CLEO conference, 1997 illustrates.
This is why the invention advantageously proposes to implement the optical control of the unipolar laser using two optical beams capable of interfering, the optically created diffraction grating making it possible to produce an optically controlled monomode unipolar laser which is easier to fabricate than that requiring etching operations.
More specifically, the subject of the invention is also a method of optically controlling a unipolar semiconductor laser, characterized in that the optical control means comprise two optical beams of the same wavelength and means for making the said beams interfere in the stack of semiconductor layers making up the laser, so as to create an interference fringe grating within the said stack.
According to one embodiment of the invention, the optical control fringe grating may be obtained from a single control laser and from means for recombining the said beams, using conventional interference methods.